Testers for the testing of circuit boards may in principle be divided into two groups, those with finger testers and those with parallel testers. The parallel testers are testers that, by means of an adapter, contact simultaneously all or at least the majority of the circuit board test points of a circuit board to be tested. Finger testers are testers for the testing of non-componented or componented circuit boards, which scan the individual contact points sequentially or serially using two or more test fingers. Serial testing with a finger tester is usually slower than parallel testing with a parallel tester.
The test fingers are generally fixed to a slide that may be moved along a cross-bar, while the cross-bar in turn is guided on and may be moved along guide rails. The slides may therefore be positioned at any desired point of a test array, which is usually rectangular. Equally, there are testers with stationary cross-bars, on which movable slides are provided. Arranged on these slides are test fingers that have a certain length and are fastened at one end pivotably to the slide. Through the swivel movement of the test finger, a certain area at right-angles to the cross-bar may be scanned. With both types of finger tester, all circuit board test points of a circuit board to be tested can be contacted and tested.
A finger tester is described in EP 0 468 153 A1 and a method of testing circuit boards using a finger tester is described in EP 0 853 242 A1.
The key factor enabling a finger tester to be successful in the market is the speed with which a circuit board to be tested is tested. In order to accelerate this test rate, special test methods (e.g. EP 0 853 242 A1 and the corresponding U.S. Pat. No. 5,977,776) or special test probes (e.g. U.S. Pat. No. 5,804,982 or U.S. Pat. No. 6,344,751) have already been developed. A test probe of this kind for the rapid contacting of a circuit board test point of a circuit board to be tested is disclosed by U.S. Pat. No. 5,113,133.
With conventional finger testers, two calibration processes are performed before the actual testing of the circuit board to be tested. In a first testing process, the test heads, each comprised of a slide and a contact finger, are calibrated with reference to the tester, and in a second calibration process, the computer aided design (CAD) data of a circuit board to be tested are brought into agreement with an actual circuit board that is inserted in the tester.
In the first calibration process, a calibration plate is inserted in the tester. This calibration plate is a large circuit board on the surface of which a grid is formed by means of the conductor paths. The individual conductor paths are arranged relative to one another with a spacing of, for example, 1–3 centimeters (cm). Only in the edge zone of this calibration plate is a very wide conductor path formed, which is in electrical contact with all other conductor paths. In the calibration process, one of the test fingers is placed on the wide conductor path, and the intersection points of the grid are scanned using another test finger. When an intersection point of the grid is touched, the coordinates of the test finger and the slide are stored, and correlated with the coordinates of the corresponding intersection point of the calibration plate. The location of the individual intersection points is effected by a step-wise approach to the intersection point, until a conductive connection is made across the two contact fingers and the conductor paths of the calibration plate. This step-wise approach may be made for example in a circular or spiral manner. For each test measurement, this involves the test finger being lowered onto and then lifted up from the calibration plate. Some ten test measurements may be necessary to locate a grid point. If all grid points off the calibration plate have been detected successfully then, for the individual sections of the test area, different correction values for driving the test fingers of the tester are determined and stored. These correction or calibration values make it possible for a test finger to be driven precisely in a coordinate system of the tester, and for it to be placed in the test area.
The second calibration process is generally performed for each circuit board to be tested. In this process, the circuit board of the batch is inserted in the tester and then, using the test fingers, prominent circuit board test points of the pattern of circuit board test points to be tested are detected and their position in the tester determined. When the circuit board test points have been detected, then the CAD data of the circuit board test points of the circuit board to be tested can be brought into agreement with the coordinates of the physical circuit board, i.e., the distortions and misalignments of the circuit board test point pattern determined as typical for a batch are recorded.
When both calibration processes have been completed, then the circuit board test points of the circuit board to be tested may be successfully approached, and contacted by means of the test fingers.
The first calibration process is necessary each time the tester is set up afresh, e.g., if a test finger has been replaced or if the tester is reconfigured at the start of a working day. The first calibration process is also performed each time the ambient temperature changes by, for example, more than 3° C.
Both calibration processes take up a considerable amount of time, which cannot be used for actual testing of circuit boards. Consequently, in the testing of circuit boards, these calibration processes have an adverse effect on throughput.
WO 92/11541 discloses an imaging system for a device for the testing of circuit boards. Similarly to an X-Y recorder, this imaging system has a movable cross-bar, on which is mounted a test head with a vertically movable test needle. Mounted next to the test needle is an imaging device comprising a lens and a charge coupled device (CCD) element. The image generated by the imaging device is displayed on a monitor. With the aid of the image displayed on the screen, an operator can control the test head in such a way that during a teach-in process, he traces all of the contact points to be tested and programs the relevant coordinates. During the test, the device automatically traces the individual contact points and contacts them with the test needle.